Field of the Invention and Related Art Statement
The present invention relates to a load detecting mechanism for use in a scale, particularly a small top loading scale.
FIGS. 1A and 1B are perspective and side views, respectively of a known typical top loading scale. The load detecting mechanism is fixed on a base plate 1 made of rigid material. On the base plate 1 there are formed a pair of root portions 2A and 2B to which cylindrical posts 3A and 3B having a given length are secured by means of screws. Upper ends of the posts 3A and 3B are coupled with each other by a reinforcing plate 4. The top loading scale further comprises upper and lower V-shaped leaf springs 5 and 6. Free ends of leg portions 5A and 5B of the upper V-shaped leaf spring 5 are clamped between top ends of the posts 3A and 3B and the reinforcing plate 4, and free ends of leg portions 6A and 6B of the lower V-shaped leaf spring 6 are clamped between the root portions 2A and 2B and lower ends of the posts 3A and 3B. The leg portions 5A and 5B of the upper leaf spring 5 are extended in parallel with the leg portions 6A and 6B of the lower leaf spring 6, respectively. Base portions of the V-shaped leaf springs 5 and 6 are secured to upper and lower ends of a vertical link 7, respectively. In this manner, the vertical link 7, leg portions 5A and 6A of upper and lower leaf springs 5 and 6 and post 3A constitute a parallelogram link mechanism, and similarly the vertical link 7, leg portions 5B and 6B and post 3B form a parallelogram link mechanism. Thus the loading system forms a Roberval mechanism and the vertical link 7 is supported by the leaf springs 5 and 6 movably in the vertical direction approximately.
As clearly illustrated in FIG. 1B an L-shaped member 9 is secured to the base plate 1, and an upper end of a load sensor 11 is connected to a free end of the L-shaped member 9, a lower end of the load sensor 11 being coupled with a projection 10 integrally formed with the vertical link 7. Further, a top weighing pan 12 is secured to the upper end of the vertical link 7 by means of a pan receiving rod 8. In the V-shaped leaf springs 5 and 6 there are formed flexures 13 at positions near the posts 3A, 3B and vertical link 7 by locally thinning a thickness of the leaf springs.
In the known top loading scale having the construction explained above, when a load W is correctly applied to the top weighing pan 12 at its center as shown by an arrow A in FIG. 1B, the load W is transferred to the load sensor 11 and can be detected correctly. However, when a load W' is applied to the weighing pan 12 at a postion deviated from its center as illustrated by an arrow B, the load W' is no longer transferred correctly to the load sensor 11 and a so-called shift error might occur. This is mainly due to the fact that the Roberval mechanism could not maintain its ideal parallelogram shape, because the loading mechanism is formed by assembling a plurality of components or parts. When a load is applied at an off-center position or an excessively large load is applied, there might be introduced twisting and bending moments in various components such as root portions 2A, 2B, posts 3A, 3B, reinforcing plate 4, L-shaped member 9 and base plate 1, so that the load detecting mechanism might be subjected to complicated elastic deformation and strain.
In the known load detecting mechanism, in order to reduce the above explained shift error, the various components are adjusted and assembled precisely. However, it is almost impossible to reduce the shift-error in all directions over a wide range of loading size. For instance, even if the shift-error in a front and rear direction can be reduced satisfactorily, the shift-error in a right and left direction could not be decreased to an allowable value. Further, although the shift-error could be reduced effectively for a small load up to 1 kg when the load larger than 3 kg is applied, the shift-error might be increased to an inpermissible large value. Moreover, the components are made of various materials having different thermal capaciticies, heat conductivities and thermal expansion coefficients, and therefore the shift-error fluctuates in accordance with the ambient temperature change and time passage.
It is apparent that since the known load detecting mechanism is composed of the various components, the assembling work becomes very cumbersome and requires a long time. Therefore, the cost of the known load detecting mechanism is liable to be high.